The importance of the rostral ventromedial medulla (RVM) in pain modulation is well established, and several independent lines of evidence support a major contribution of this brainstem region to opioid analgesia. Local microinjection of morphine into the RVM produces an antinociception, and evidence recently obtained using iontophoretic application of morphine indicates that the direct action of this compound is exerted on one identifiable subpopulation of RVM neurons. This group of neurons, termed "on-cells", show a naloxone-reversible decrease in firing rate when morphine is applied by iontophoresis. On-cells are characterized by an abrupt increase in firing just prior to the occurrence of nocifensive reflexes such as the TF response, and are likely to have a permissive or even facilitating effect on nociception. These neurons invariably become silent following systemic, intrathecal or periaqueductal gray (PAG) administration of morphine in doses sufficient to inhibit the TF. These observations point to a key role for the on-cell as a primary target of opioid action in RVM, with depression of on-cell firing likely to make a significant contribution to opioid antinociception. The overall goal of the present proposal is to examine the mechanisms underlying opioid-induced depression of on-cell firing by identifying the neurotransmitters involved in controlling the activity of this cell class. The strategy will be to use iontophoretic techniques to identify the neurotransmitters mediating inputs to RVM on-cells that are activated or blocked by administration of morphine in the PAG or intrathecally. The depression of RVM on-cell firing that is produced by PAG morphine microinjection is most likely mediated by activation of an inhibitory input to the on-cell. The contributions of 5HT and GABA to this effect will be examined using iontophoretic application of receptor antagonists to block this PAG opioid-induced depression. The possibility that an excitatory amino acid neurotransmitter contributes to the TF-related activation of the on-cell will also be investigated, as this burst of activity is blocked by morphine applied intrathecally. These studies will further our knowledge of the neural substrate of opioid analgesia, and contribute to a clearer view of the central mechanisms that modulate pain transmission. The understanding of brainstem modulatory circuitry derived from these studies should advance our ability to explain, predict, and manipulate pain modulation.